Experimental studies on microstructural and corrosion
characterisation of additively manufactured AISI 316 L
austenitic stainless steel
Abstract
Microstructure and pitting corrosion behaviour of additively manufactured wall of AISI 316L
was investigated. The results indicated that weld zone, including ferrite and austenite phases,
was mainly composed of columnar dendrites. No obvious re-passivation was shown in as-
welded and thermally aged samples. Pitting occurred in both the as-weld and heat treated
samples. However, the pitting potential was less in thermally aged sample signifying less
pitting resistance.
Keywords: Wire-Arc Additive Manufacturing; Corrosion, Microstructure, Stainless steel
Introduction
Austenitic stainless steel is widely used as the structural material in many fields for its
outstanding mechanical and corrosion properties. [1] Welding is extensively selected
to connect these structural materials, and then the microstructure variation exists in the
weld joint.[2] It is believed that the microstructure discrepancy results in different local
corrosion behaviour such as pitting corrosion and intergranular corrosion and stress
corrosion cracking during service.[3]
AISI 316 L austenitic stainless steel is used as implant materials to make such devices
as artificial joints, bone plates, stents and so on, thanks to its favourable combination
of mechanical properties, corrosion resistance, satisfactory biocompatibility and
relatively low cost compared with other metallic biomaterials [4].
However, after its implantation in human bodies, sometimes mechanical failure occurs
and tissue inflammation may happen, which makes safety become dissatisfactory and
insufficient for its biomedical applications [5]. Because of the high concentration of
Cl− and the temperature range of 36.737.2 °C, the human body fluid is considered a
severely corrosive environment and localized corrosions such as pitting, crevice
corrosion and fretting fatigue are probable to appear on AISI 316L steel [6].
Metal ions such as iron, chromium and especially nickel could be released and thus
cause toxicity to the body and deteriorate the AISI 316L biocompatibility [7].
Therefore, improvement of the biocompatibility of AISI 316L steel, especially its blood
compatibility when used in the vascular environment, can be beneficial to its safe use
in the human body.
The objective of this work is to investigate the microstructure and pitting corrosion
behaviour of AISI 316L stainless steel additively manufactured using WAAM.
Background and Literature Review
Literature review
According to the previous works, the weld joint is mainly composed of weld zone, heat-
affected zone (HAZ), and base metal. Normally, it is accepted that weld zone possesses
a duplex structure including ferrite and austenite phases and the ferrite in the weld zone
reduces the hot cracking susceptible of the weld joint, while Kwok [8] also found that
the presence of ferrite in laser-welded SS304 stainless steel joint contributed to a
decrease in pitting corrosion resistance. The HAZ, caused by thermal cycle during
welding process, exhibits to be more susceptible to pitting corrosion compared with
base metal. Second phase precipitation, recrystallization, and residual stress are
proposed as the accelerating factors for pitting corrosion resistance, degradation of
HAZ and it is reported that HAZ is the most sensitive zone for pitting corrosion.
However, Lu [9] found that weld zone had a lower breakdown pitting potential than
HAZ under gas tungsten arc welding process, confirming the existence of pitting
corrosion performance discrepancy of weld zone under different welding methods and
conditions.
Stainless steel
Stainless steel is an alloy of iron and several other elements (such as nickel, chromium,
molybdenum, and carbon) and due to these elements it is more resistant to corrosion
than plain iron or steel (which is simply iron and carbon).These stainless steel elements,
such as nickel, chromium, and other additives, give it a passive oxide layer that resists
the formation of rust and creates a shiny, reflective surface. This passive layer has the
unique ability to repair itself. The shiny surface of stainless steel is very difficult to
tarnish compared to plain steel, hence why it is called “stainless” steel. It is considered
environmentally neutral and inert that is why it has longevity because it does not leach
compounds that could modify its composition when in contact with elements like water
and air. The four grades of stainless steel have been classified according to their
material properties and welding requirements:
Austenitic stainless steels
Ferritic stainless steels
Martensitic stainless steels
Duplex stainless steels
Austenitic stainless steel
Austenitic stainless steels has a faced-centered cubic (FCC) crystal structure and a grain
structure consisting of austenite. It has a Cr-level of 16 - 25 % and an addition of up to
35 % Ni to stabilize the austenitic structure. Since so much Ni is added it also makes
the austenitic stainless steels more expensive than ferritic stainless steels. They are non-
magnetic and have a good formability and weldability. The temperature use-span is
wide, from temperature below 123 K up to red-hot temperatures. Austenitic stainless
steels has a wide use and it often used in aircraft applications, food and dairy industry
and pulp and paper manufacturing.
Ferritic stainless steel
Ferritic stainless steels has a body-centred cubic (BCC) crystal structure and a grain
structure consisting of ferrite. The Cr-level spans from 10.5 % to greater than 25 %.
Their low cost made it grow in use and they are well suited for use as light-gauge sheets.
They are ferromagnetic, contains a low amount of carbon and are for some alloys poor
for welding in thicker-walled pieces due to the formation of brittle martensite. They
cannot be hardened by heat treatment, but strengthen by annealing. Ferritic stainless
steels has a wide use in automotive exhaust systems and kitchen applications
Martensitic stainless steel
Martensitic stainless steels have either a martensitic α - or ǫ-phase. Martensite forms
at high cooling rates, when the transformation of austenite happens diffusionless. They
have a low corrosion resistance due to the low amount of alloying elements used to
keep the martensitic phase stable. This reduces the corrosion resistance, but they still
fill a use since they have very good mechanical properties such as high strength and
hardness. They are perfect for high-temperature applications where a strong and stable
material is needed.
Duplex stainless steel
Duplex stainless steel is the newest type of stainless steel alloy. It is a combination of
austenitic and ferritic structure to achieve a high strength. They have a high corrosion
resistance due to the amount of Cr being more than 20 %. They are less expensive than
austenitic stainless steels due to the low amount of Ni used. They are often used in -
100 300 C applications where austenitic stainless steels have been used before and
where high strength is needed.
Interpretation of the Microstructure of Steels
Microstructure is the very small scale structure of a material, defined as the structure
of a prepared surface of material as revealed by an optical. The microstructure of a
materials can strongly influence physical properties such as strength, toughness,
ductility, hardness, corrosion resistance, high/low temperature behaviour or wear
resistance. These properties in turn govern the application of these materials in
industrial practice.
Figure 1-Lattice of iron and other alloying elements [10]
Steel properties come from their microstructural phases, small gaps between atoms,
called interstices, are where small elements like carbon and nitrogen fit. As the alloying
increases, the straining in the atomic lattice increases, requiring more force to deform
the work piece, thereby increasing the strength.
When a very small fraction of the interstices in between the iron lattice is occupied by
carbon atoms, this interstitial-free steel is said to have a microstructure of ferrite. Ferrite
has a body-centered cubic (BCC) crystal structure. Ferrite is a microstructural phase
that is soft, ductile, and similar to pure iron. There is a limit on how much carbon can
fit in the gaps in the ferrite structure: 0.02 percent carbon at 1,340 degrees F (725
degrees C), but dropping to 0.006 percent (60 PPM) carbon at room temperature.
The gaps are a little larger in a phase known as austenite, which has a face-centered
cubic (FCC) crystal structure. At around 2,100 degrees F (1,150 degrees C), up to 2
percent carbon can fit into the austenite microstructure.
As the steel slowly cools from this temperature and carbon is forced out of solution,
the austenite transforms into a combination of ferrite and another phase
called cementite, also known as iron carbide, which has the chemical composition of
Fe
3
C. The amount of cementite that forms is a function of how much carbon is in the
steel. Because ferrite cannot contain more than about 60 PPM carbon at room
temperature, the rest of the carbon winds up as cementite.
Unlike ferrite, cementite has the characteristics of a ceramic: very hard and brittle, with
low toughness and little resistance to crack initiation and propagation. The mixture of
ferrite and cementite is called pearlite, named because it looks like mother of pearl
under a microscope, with alternating layers of ferrite and cementite.
With faster cooling, different dynamics occur. Above a critical cooling, the excess
carbon of the FCC austenite does not have time to diffuse out of the crystal structure
and form cementite. Instead, the carbon is trapped in with the now nearly pure iron and
forced into the interstitial locations that are not large enough to accommodate the
carbon atoms. This distorts and strains the crystal matrix into a body-centered
tetragonal (BCT) structure, forming a hard phase called martensite.
At higher carbon levels, more carbon is frozen into the BCT structure, further straining
the crystal matrix. This is why the hardness of martensite increases with carbon level.
The volume of the BCT martensite structure is larger than that of the FCC austenite, so
the freshly transformed martensite is compressed by the surrounding matrix.
If martensite is heated, carbon has the opportunity to diffuse out from the BCT
structure, reducing the distortion of the crystal matrix, leading to decreased hardness
and increased toughness. This heat treatment produces a microstructure of ferrite and
iron carbide (Fe
3
C) called tempered martensite.
Bainite is another microstructure that can form when austenite is cooled. It typically
consists of a combination of ferrite, cementite, and retained austenite. Because the
cooling rate to form bainite is slower than the cooling rate needed to form martensite,
carbon has some opportunity to diffuse out of the FCC austenite, allowing for the
formation of BCC ferrite. Bainitic microstructures have the best balance of strength
and ductility. The cooling rate is fast enough to increase the strength, while the rounded
hard microstructural constituents are not as prone to crack initiation and propagation as
if they were flat and elongated.
Figure 2-Schematic diagram showing time-temperature transformation curve for plain
carbon eutectoid steel [10]
Weld metal solidification
During the solidification of a pure metal the solid-liquid interface is usually planar,
unless severe thermal undercooling is imposed. During the solidification of an alloy,
however, the solid-liquid interface and hence the mode of solidification can be planar,
cellular, or dendritic depending on the solidification condition and the material system
involved.
Figure 3-Basic solidification modes: (a) planar solidification, (b) cellular
solidification, (c) columnar dendritic solidification, (d) equiaxed dendritic
solidification [11]
Typical microstructures resulting from the cellular, columnar dendritic, and equiaxed
dendritic modes of solidification in alloys are shown below-
Figure 4-Nonplanar solidification structure in alloys. (a) Transverse section of a
cellularly solidified PbSn alloy, (b) Columnar dendrites in a Ni alloy, (c) Equiaxed
dendrites of a MgZn alloy, (d) Three-dimensional view of dendrites in a Ni-base
super-alloy [11]
Corrosion in materials
Types of corrosion
Galvanic Corrosion
Galvanic corrosion is also known as bimetallic corrosion, as the name suggests
two different types of metal, when two dissimilar metals kept together directly
or indirectly, in one of the metals corrosion rate occurs at a faster pace and the
other metal remains unaffected. There is basically a galvanic couple that forms
between the metals and one become anodic and the other cathodic. The main
factor which plays the role is the magnitude of the potential difference between
the metals.
High-temperature Corrosion
Generally above 500ºC high-temperature corrosion takes place. Although there
are many things that occur at high temperatures one of the main things is
oxidation. Because of this the loss in material changes in mechanical properties
because of the change in the microstructure of the alloy, and many more occurs.
Intergranular Corrosion
In this corrosion, grain boundaries are under attack, and carbides are formed,
further propagating for higher corrosion. The cause of this corrosion, in general,
is improper heat treatment. In austenitic stainless steel, chromium carbides can
become precipitated at the grain boundaries which reduces the individual
chromium concentration and makes boundaries prone to corrosion.
Localized Corrosion Pitting
The most common form of localized corrosion is pitting corrosion. As the name
suggests it forms a hole of a small diameter that penetrates into the surface,
generally visible to naked eyes and the remaining surface is intact. The leading
causes of pitting corrosion are lack of passive film, high aggressive medium in
which the metal exists, poor coating application, and much more.
Localized Corrosion Crevice
One of the highly penetrating types of localized corrosion is a crevice, which
occurs due to a small gap in the surface of the metal for example cracks, seams,
small spaces that occur during manufacturing, and much more.
Localised Corrosion Filiform
Corrosion that occurs under the coated surface on the main metal. Occurs due
to the defects in the coating of the surface when some external elements attack
the main metal because of the defect in the protective layer.
Stress Corrosion Cracking (SCC)
The corrosion occurs due to the tensile, bending, and torsion stress generally
often at rapid high temperatures. The only prevention for SCC is to choose
proper fabrication with suitable material and suitable thermal and mechanical
stresses.
Electrochemical techniques
The condition of corrosion in a concrete or metal element is determined through
electrochemical corrosion testing. Electrochemical corrosion testing, which is based on
electrochemical theory, evaluates corrosion damage and, when possible, determines
corrosion rates. All corrosion is an electrochemical reaction involving oxidation and
reduction. To assess the corrosion characteristics of metals and metal components in
conjunction with various electrolyte solutions or soil conditions, controlled
electrochemical experimental approaches can be utilised. Each metal/solution system
has its own corrosion properties. The ASTM/NACE methods, Cyclic Polarization,
Impedance Spectroscopy, linear polarisation tests, electrochemical testing for hydrogen
permeation, real-time corrosion forecasts for materials, and others are among the
electrochemical techniques.
Cyclic Potentiodynamic Polarization (CPP)
CPP test was introduced for the first time in the 1960s. CPP measurements should be
carried out according to the defined ASTM standards (F2129, G61). It is widely used
to determine resistance to localized corrosion or degradation rate in a short time. Thus
this technique is applicable as a method for prediction of localized corrosion also
beneficial for alloys that are passivized spontaneously and underwent localized
corrosion.
General shape of CPP curve is as follows; after passing through the region of active
corrosion, the current density decreases to a critical potential, called the “Flade
potential” or “primary passivation potential”. This decrease is due to the formation of
the passive layer on metal surface.
The passive current density is the current density in the passive region. With further
increase in the potential in the passive region, a rapid rise in the anodic current can be
detected. This rise is due to either the evolution of oxygen by the decomposition of
water or breaking the passive film and localized corrosion. If the increase of current
density is due to the decomposition of water and evolution of oxygen gas, the region is
called “transpassive region”
Figure 5-Schematic illustration of CPP curve [12]
Corrosion control techniques
The following methods are used to protect metals against corrosion:
Selection of the right material of construction, Surface coating, use of Inhibitors, proper
equipment design and electrical protection
1. Barrier coatings
One of the easiest and cheapest ways to prevent corrosion is to use barrier
coatings like paint, plastic, or powder. Powders, including epoxy, nylon, and
urethane, adhere to the metal surface to create a thin film. Plastic and waxes are
often sprayed onto metal surfaces. Paint acts as a coating to protect the metal
surface from the electrochemical charge that comes from corrosive compounds.
Today’s paint systems are a combination of different paint layers that serve
different functions. The primer coat acts as an inhibitor, the intermediate coat adds
to the paint’s overall thickness, and the finish coat provides resistance to
environmental factors.
The biggest drawback with coatings is that they often need to be stripped and
reapplied. Coatings that aren’t applied properly can quickly fail and lead to
increased levels of corrosion. Coatings contain volatile organic compounds, which
make them hazardous to people and the environment.
2. Hot-dip galvanization
This corrosion prevention method involves dipping steel into molten zinc. The
iron in the steel reacts with the zinc to create a tightly bonded alloy coating which
serves as protection. The process has been around for more than 250 years and has
been used for corrosion protection of things like artistic sculptures and playground
equipment.
Unfortunately, galvanization can’t be done on-site, meaning companies must
pull equipment out of work to be treated. Some equipment may simply be too large
for the process, forcing companies to abandon the idea altogether. In addition, zinc
can chip or peel. And high exposure to environmental elements can speed up the
process of zinc wear, leading to increased maintenance. Lastly, the zinc fumes that
release from the galvanizing process are highly toxic.
3. Alloyed steel (stainless)
Alloyed steel is one of the most effective corrosion prevention methods around,
combining the properties of various metals to provide added strength and resistance
to the resulting product. Corrosion-resistant nickel, for example, combined with
oxidation-resistant chromium results in an alloy that can be used in oxidized and
reduced chemical environments. Different alloys provide resistance to different
conditions, giving companies greater flexibility.
Despite its effectiveness, alloyed steel is very expensive.
4. Cathodic protection
Cathodic protection protects by electrochemical means. To prevent corrosion,
the active sites on the metal surface are converted to passive sites by providing
electrons from another source, typically with galvanic anodes attached on or near
the surface. Metals used for anodes include aluminum, magnesium, or zinc.
While cathodic protection is highly effective, anodes get used up and need to
be checked and/or replaced often which can drive up costs of maintenance. They
also increase the weight of the attached structure and aren’t always effective in
high-resistivity environments.
Methodology and experimental details
Methodology
Figure 6- Flowchart showing methodology opted in the present work
Experimental details
The experimental details of the microstructure evaluation and corrosion examination
of WAAM manufactured AISI 316L welds is given as follows-
Microstructure Evaluation-
For carrying out the microstructural evaluation of the fabricated welds, samples were
cut from the fabricated walls and the surfaces were sanded and polished down up to
3000 grit of emery paper .These samples were etched using carpenter’s etchant. Each
sample was immersed in the etchant at 20
o
C and then water rinsed.
Table 1- Carpenter's etchant composition used in the present examination
Carpenters
Stainless
Steel Etch
FeCl
3
8.5 grams
Immersion
etching at
20 degrees
Celsius
For etching
duplex and
300 series
stainless
steels
CuCl
2
2.4 grams
Hydrochloric acid
122 ml
Nitric acid
6 ml
Ethanol
122 ml
An inverted type optical metallurgical microscope was used to examine microstructure
on etched samples.
Corrosion Examination-
Corrosion characterisation of the welds was done using a potentiostat (Gamry
Instruments, model: Reference 600) supported by Gamry framework software. For
conducting cyclic potentiodynamic polarization (CPP) test in a paracell at room
temperature, where three electrodes, namely a saturated calomel electrode (SCE) as a
reference electrode, graphite counter electrode and working electrode (weld test
sample), were used. Each sample was exposed to an electrolytic solution of 0.5 M
H
2
SO
4
(sulfuric acid) + 0.5 M NaCl (sodium chloride).
Results and Discussions
A: Microstructural Studies of Welds
Weld metal microstructures of the test samples are presented in Figures 6(a) through
(b). The microstructure of the HAZ is characterised with dendrites. The weld zone is
solidified to form austenite + ferrite matrix in additively manufactured AISI 316L. The
figure 6(a) represents individual fusion boundaries of additively manufactured layers.
Each HAZ and fusion zone is clearly visible in the microstructure. The grains are
closely packed in the HAZ region and are loosely spaced in the weld zone.
Figure 7-Microstucture revealed using carpenters etchant
Figures 7(a) to 7(c) shows the microstructure of WAAM wall taken at different
positions. The figures reveal that in all the three locations austenite matrix was present
throughout the additively manufactured wall
Figure 8- Image showing microstructures of additively manufactured WAAM wall (a)
At the bottom-layer, (b) At the middle layer, (c) At the top layer
B: Corrosion behaviour
Pitting potential - The least positive current and voltage at which pits develop or grow
on a metallic surface. This is the electrochemical potential in a given environment
above which a corrosion pit initiates on a metallic surface
Corrosion potential - Corrosion potential is a mixed potential (also an open-circuit
potential or rest potential) at which the rate of anodic dissolution of the electrode equals
the rate of cathodic reactions and there is no net current flowing in or out of the
electrode.
Repassivation Potential - Pits repassivate below the repassivation potential, denoted
by E
rp
CASE 1: As-Welded Condition (CPP)
Table 2-CPP test conditions for as-welded test sample
Initial E (V)
-0.25
Apex E (V)
1.5
Final E (V)
0
Forward Scan (mV/s)
1
Reverse Scan (mV/s)
1
Sample Period (s)
1
Apex I (mA/cm^2)
10
Density (g/cm^3)
7.805
Init. Delay Time (s)
300
Results
Pitting potential - 325.6 mV
Corrosion potential -210.1 mV
Re-passivation Potential - NIL
Figure 9- Cyclic potentiodynamic polarisation test results for as-welded sample
Figure 10- Weld sample after CPP test (significant pits observed on the surface)
CASE 2: Thermally Aged Condition (CPP)
Thermal ageing condition: 750
o
C for 24 hrs and water quenched
Table 3-CPP test conditions for thermally aged test sample
Initial E (V)
Apex E (V)
Final E (V)
Forward Scan (mV/s)
Reverse Scan (mV/s)
Sample Period (s)
Apex I (mA/cm^2)
Density (g/cm^3)
Init. Delay Time (s)
Results
Pitting potential [-190.1 mV]
Corrosion potential 262.3 mV
Re-passivation Potential - NIL
Figure 11-Cyclic potentiodynamic polarisation test results for thermally-aged sample
Figure 12-Thermally aged weld sample after CPP test (significant pits observed on
the surface)
Figure 13- Curve showing overlay plot of cases considered
Few noteworthy findings from the graph are-
Percentage Difference Pitting potential - 41.6%
Percentage Difference Corrosion potential - 24.8%
Percentage Difference Re-passivation Potential - NIL
Figure 14-Image showing sample preparation (a) WAAM wall manufactured for
experimentation, (b) Samples cut for examination, (c) Polished sample
Figure 15- Equipment used in the present work (a) Muffle furnace used for thermally
ageing the samples, (b) Potentiostat used for conducting corrosion tests
Figure 16- (a) Precision balance used for measuring the proportions of the salt for
electrolyte formation, (b) Electrolytic solution made, (c) Experimental setup
Conclusions
The microstructure and pitting behaviour of AISI 316L stainless steel WAAM wall was
investigated. Microstructures in the additively manufactured wall was examined by
OM, the pitting corrosion of the as welded and heat treated samples of the stainless
steel wall was investigated by potentiodynamic polarization tests using the neutral
chloride electrolyte 17.5 gm NaCl in 500 mL solution. The results were obtained as
follows:
1. Weld zone, exhibited a dual structure including ferrite and austenite phases and
was composed of columnar dendrites primarily.
2. Compared with weld zone, grain refinement occurred in the HAZ.
3. A significant decrease of 41.6% and 24.8% in pitting potential and corrosion
potential after thermal aging of the specimen was observed. Higher pitting
potential indicates higher pitting resistance.
4. In both the specimen as-welded and thermally aged the re-passivation potential
was not observed.
5. In thermal aged specimens the amount of pits were significantly higher as
compared to as-welded condition.
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